MOSCOW — An estimated 120,000 Armenians rallied today to protest the loss of part of their homeland, the second such gathering in recent days and one of the biggest unofficial demonstrations ever reported in the Soviet Union.

Moscow dissidents Alexander Ogorodnikov and Tamara Grigoryants said the streets around the opera house in Yerevan, capital of Soviet Armenia, were mobbed with people demanding reattachment of a small mountainous region to the Armenian republic.

Grigoryants said the region in the Caucasus Mountains, named Nagorno-Karabakhskaya, was deeded to the neighboring Soviet republic of Azerbaijan in the 1920s, even though the majority of its inhabitants are Armenians.

A woman who said she lives on the outskirts of Yerevan, about 1,100 miles south of Moscow, said by telephone that demonstrations also occurred in Nagorno-Karabakhskaya itself during the weekend.

In a highly unusual step, the government acknowledged the earlier protest today by noting that a "breaching of public order" occurred and that the demonstrators "contradict the interests of the working people."

Tass Carries Dispatch

The government comment came in a dispatch carried by the official press agency Tass.

"Breaching of public order was provoked as a result of irresponsible calls by extremist individuals," Tass said, referring to the unrest in Nagorno-Karabakhskaya.

It added that the Communist Party Central Committee "holds that the actions and demands directed at revising the existing national and territorial structure contradict the interests of the working people in Soviet Azerbaijan and Armenia and damages inter-ethnic relations."

The Central Committee urged residents "not to yield to provocations by nationalist elements" and warned that all measures would be taken to "safeguard public order," Tass said.

Grigoryants said that earlier this month, the local government council asked that the disputed region become part of Armenia. The request was rejected by the Communist Party Central Committee, she said.

Grigoryants and Ogorodnikov said in separate telephone interviews that protest demonstrations in Yerevan's center had lasted from late Monday night through today.

Ogorodnikov, a former political prisoner, said 70,000 people marched through the streets late Monday to demand that Nagorno-Karabakhskaya become part of Armenia.

Party Chief Asks Calm

Armenian Communist Party chief Karen S. Demirchyan went on television Monday night to ask residents to be calm and to assure Armenians that the question of the autonomous region's future was being considered, Ogorodnikov said.

Thousands of people who had congregated outside the opera whistled when they heard Demirchyan's words and called the party boss a liar, he said.

Ogorodnikov said that by this afternoon, the crowd outside the Yerevan opera house had swelled to 120,000, and that leaders of the protest issued an ultimatum to the Armenian Supreme Soviet demanding that parliament convene.

Grigoryants said if the parliament does not agree to meet, a general strike will probably be called.

Both dissidents said the police behaved with restraint.

Since the 1915 Turkish invasion of Armenia in which at least 1.5 million Armenians are said to have been killed, there have been more Armenians living abroad and in other parts of the Soviet Union than in their historic homeland south of the Caucasus.

Yerevan is home to about one-third of Armenia's 3.3 million people, 90% of whom are ethnic Armenians.